Magnetic coating compositions containing ferromagnetic metal powder and/or ferromagnetic alloy powder, which are to be used in the production of magnetic recording materials, are usually prepared by fully dispersing such magnetic materials and polymeric binders along with suitable organic solvents by means of a dispersing machine, e.g., a ball mill and a sand mill. If necessary, other materials may be added such as antistatic agents, lubricants, abrasives, hardeners, and so forth. The degree of dispersion of the magnetic coating composition is closely related to or responsible for the surface smoothness, orientation ratio, and so forth of the magnetic recording material produced therefrom. Further, it is the most important factor relating to the electromagnetic transforming characteristics of the magnetic recording material. Therefore, in order to produce magnetic coating compositions having much higher degree of dispersion, it is quite important to select suitable combinations of magnetic materials and polymeric binders.
Furthermore, the durability of the magnetic recording material varies greatly with the combination of magnetic material and polymeric binders. Thus, also from a viewpoint of durability, it is very important to select suitable combinations of magnetic materials and polymeric binders.
Heretofore, as such polymeric binders, vinyl chloride/vinyl acetate-based copolymers, polybutyral resins, phenoxy resins, thermoplastic polyurethane resins, acrylonitrile/butadiene-based copolymers, synthetic rubbers, etc. have been used alone or in combination with each other.
When, however, these polymeric binders are used in the preparation of magnetic coating compositions containing ferromagnetic metal powder or ferromagnetic alloy powder, such metal type magnetic materials are insufficiently dispersed in the polymeric binders. This is because metal type magnetic materials have very poor dispersibility compared with other oxide type magnetic materials. Thus, magnetic recording materials produced from these metal type magnetic coating compositions have disadvantages in that the surface properties are poor and the orientation ratio is low.
On the other hand, magnetic coating compositions using conventional oxide type magnetic materials and nitrocelluloses, e.g., HIG1/2 and RS1/2 are described in Japanese Patent Publication Nos. 48,003/77, 48,004/77, 4,122/75, 48,126/74, 46,921/74 and 46,921/72, and Japanese Patent Application (OPI)Nos. 43,405/77, 44,904/76 and 67,605/75. These magnetic coating compositions are superior particularly in dispersion characteristics. However, when the metal type magnetic materials are used in place of such oxide type magnetic materials, the resulting magnetic coating compositions are seriously increased in viscosity or become gel, as reported in Japanese Patent Publication No. 22,063/72. The increase in viscosity is caused by the surface activity of the metal type magnetic materials being completely different from those of the conventional oxide type magnetic materials. Accordingly, it is not possible to obtain a product in a liquid paint form.